Methods, systems, and computer program products for measuring the density of material including a non-nuclear moisture property detector

ABSTRACT

The subject matter described herein includes methods, systems, and computer program products for measuring the density of a material. According to one aspect, a material property gauge includes a nuclear density gauge for measuring the density of a material. The nuclear density gauge includes a radiation source adapted to emit radiation into a material and a radiation detector operable to produce a signal representing the detected radiation. A first material property calculation function is configured to calculate a value associated with the density of the material based upon the signal produced by the radiation detector. The material property gauge further includes an electromagnetic moisture property gauge configured to determine a moisture property of the material. The electromagnetic moisture property gauge includes an electromagnetic field generator configured to generate an electromagnetic field where the electromagnetic field sweeps through one or more frequencies and penetrates into the material. The material includes at least one of a pavement material, aggregate base material, concrete, and a soil material. An electromagnetic sensor is configured to determine a frequency response of the material to the electromagnetic field across the one or more frequencies. A second material property calculation function is configured to correlate the frequency response to a moisture property of the material and to calculate a value representing the moisture property. The material property gauge further includes a third material property calculation function configured to determine a material property of the material based on the value associated with the density of the material and the value representing the moisture property of the material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/910,745, filed Oct. 22, 2010 now U.S. Pat. No. 7,928,360, which is acontinuation of U.S. patent application Ser. No. 12/534,739 (now U.S.Pat. No. 7,820,960), filed Aug. 3, 2009, which is a continuation of U.S.patent application Ser. No. 11/512,732 (now U.S. Pat. No. 7,569,810),filed Aug. 30, 2006, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/712,754, filed Aug. 30, 2005, and U.S.Provisional Patent Application Ser. No. 60/719,071, filed Sep. 21, 2005,the disclosures of which are incorporated by reference herein in theirentireties. The disclosure of U.S. patent application Ser. No.11/513,334, filed Aug. 30, 2006, is incorporated by reference in itsentirety.

TECHNICAL FIELD

The subject matter described herein relates to measuring materialproperties. More particularly, the subject matter described hereinrelates to methods, systems, and computer program products for measuringthe density of a material including a non-nuclear moisture propertydetector.

BACKGROUND

In construction engineering, some of the most important properties ofinterest are volumetric and mechanistic properties of a bulk soil mass.In particular, there are procedures in construction engineering practicethat relate total volume V_(t), mass of water M_(W), and mass of drysolids M_(S) to the performance of a structure built on a soilsfoundation. Thus, the measurements of these properties are important forconstruction engineering.

Material density and moisture content are other important materialproperties used for design, quality control, and quality assurancepurposes in the construction industry. Some exemplary techniques formeasuring the density and moisture content of soils include nuclear,sand cone, and drive cone, as described by the American Society ofTesting and Materials (ASTM) standards D-2922, D-3017, and D-1556, andthe American Association of State Highway and Transportation Officials(AASHTO) standards T-238, T-239, T-191, and T-204. The nuclearmeasurement technique is non-destructive and calculates both the densityand the moisture content in a matter of minutes. The sand cone and drivecone measurement techniques require the moisture content test of ASTMstandard D-2216, which involves a time consuming evaporation process.The moisture content test involves heating a sample to 110° C. for atleast 24 hours.

For road construction, there is an optimum water or moisture contentthat allows for obtaining a maximum density. An exemplary density testis described in ASTM standard D-698, wherein a field sample is preparedwith different water contents, and compacted with like energy efforts.Hence, each sample has different water content, but the same compactioneffort. The densities are then measured gravimetrically in thelaboratory. The moisture content with the highest density is deemed theoptimum condition and selected as the field target. Summarily, theobjective of material compaction is the improvement of materialproperties for engineering purposes. Some exemplary improvements includereduced settling, improved strength and stability, improved bearingcapacity of sub grades, and controlling of undesirable volume changessuch as swelling and shrinkage.

In the road paving and construction industry, portable nuclear densitygauges are used for measuring the density of asphalt pavement and soils.Often, an asphalt paving material is applied on a new foundation ofcompacted soil and aggregate materials. The density and moisture contentof the soil and aggregate materials should meet certain specifications.Therefore, nuclear gauges have been designed to measure the density ofthe asphalt pavement and soils.

Nuclear density gauges typically include a source of gamma radiationwhich directs gamma radiation into the sample material. A radiationdetector may be located adjacent to the surface of the sample materialfor detecting radiation scattered back to the surface. From thisdetector reading, the density of the sample material can be determined.

These nuclear gauges are generally designed to operate either in abackscatter mode or in both a backscatter mode and a transmission mode.In gauges capable of transmission mode, the radiation source isvertically moveable from a backscatter position, where it resides withinthe gauge housing, to a series of transmission positions, where it isinserted into holes or bores in the sample material to selectabledepths.

Nuclear gauges capable of measuring the density of sample materials havebeen developed by the assignee of the present subject matter. Forexample, nuclear gauges for measuring the density of sample materialsare disclosed in U.S. Pat. Nos. 4,641,030; 4,701,868; and 6,310,936, allof which are incorporated herein by reference in their entirety. Thegauges described in these patents use a Cesium-137 (Cs-137) source ofgamma radiation for density measurements, and Americium Beryllium (AmBe)neutron sources for moisture measurements. Paving material may beexposed to the gamma radiation produced by the Cs-137 source. Gammaradiation is Compton scattered by the paving material and detected byGeiger-Mueller tubes positioned to form at least one geometricallydiffering source-to-detector relationships. The density of the pavingmaterial is calculated based upon the gamma radiation counts detected bythe respective detectors.

One difficulty to the use of nuclear density gauge is the use of aradioactive source and the associated regulations imposed by the U.S.Nuclear Regulatory Commission (NRC). The requirements for meeting NRCregulations are largely dependent on the quantity of radioactive sourcematerial used in a gauge. Thus, it is desirable to provide a nucleardensity gauge having a smaller quantity of radioactive source materialin order to reduce the requirements of the NRC for use of the gauge.

Another difficulty with nuclear gauges is the time required for making adensity measurement of material. Delays in obtaining densitymeasurements of soils during construction may delay or otherwise disturbthe construction process. Thus, it is desirable to provide a nucleardensity gauge operable to provide faster density measurements.

Accordingly, in light of the above described difficulties and needsassociated with nuclear density gauges, there exists a need for improvedmethods, systems, and computer program products for measuring thedensity of material.

SUMMARY

The subject matter described herein includes methods, systems, andcomputer program products for measuring the density of a material.According to one aspect, a material property gauge includes a nucleardensity gauge for measuring the density of a material. The nucleardensity gauge includes a radiation source adapted to emit radiation intoa material and a radiation detector operable to produce a signalrepresenting the detected radiation. A first material propertycalculation function is configured to calculate a value associated withthe density of the material based upon the signal produced by theradiation detector. The material property gauge further includes anelectromagnetic moisture property gauge configured to determine amoisture property of the material. The electromagnetic moisture propertygauge includes an electromagnetic field generator configured to generatean electromagnetic field where the electromagnetic field sweeps throughone or more frequencies and penetrates into the material. The materialincludes at least one of a pavement material, aggregate base material,concrete, and a soil material. An electromagnetic sensor is configuredto determine a frequency response of the material to the electromagneticfield across the one or more frequencies. A second material propertycalculation function is configured to correlate the frequency responseto a moisture property of the material and to calculate a valuerepresenting the moisture property. The material property gauge furtherincludes a third material property calculation function configured todetermine a material property of the material based on the valueassociated with the density of the material and the value representingthe moisture property of the material.

As used herein, the terms “sample construction material,” “samplematerial,” “construction material,” and “material” refer to any suitablematerial used in a construction process. Exemplary sample constructionmaterials include soil, asphalt, pavement, stone, sub-base material,sub-grade material, cement, agricultural soils, batch plants, concretecuring rate, concrete chloride inclusion, sodium chloride content,concrete delamination, water content, water-cement materials,alkali-silica, various soils, flexible asphalt, and any combinationthereof.

The subject matter described herein may be implemented using a computerprogram product comprising computer executable instructions embodied ina non-transitory computer-readable medium. Exemplary non-transitorycomputer-readable media suitable for implementing the subject matterdescribed herein include chip memory devices, disk memory devices,programmable logic devices, and application specific integratedcircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the subject matter described herein will now beexplained with reference to the accompanying drawings of which:

FIG. 1A is a graph of a comparison of dielectric constants of claymaterial and non-clay material over different frequencies;

FIG. 1B is a graph of dielectric constant dispersion of severaldifferent types of clays;

FIG. 1C is a graph of dielectric dispersion of the conductivity anddielectric constant of cohesive soil;

FIG. 2 is a vertical cross-sectional view of a nuclear density gauge formeasuring the density of material according to an embodiment of thesubject matter described herein;

FIG. 3 is a vertical cross-sectional view of the nuclear density gaugeshown in FIG. 2 configured in a transmission mode for measuring thedensity of a sample material according to an embodiment of the subjectmatter described herein;

FIG. 4 is a flow chart of an exemplary process by which the gauge shownin FIGS. 2 and 3 may be initialized according to an embodiment of thesubject matter described herein;

FIG. 5 is a flow chart of an exemplary process for determining detectorcounts within an energy window according to an embodiment of the subjectmatter described herein;

FIG. 6 is a vertical cross-sectional view of the nuclear density gaugeshown in FIGS. 2 and 3 for measuring the density of asphalt layersaccording to an embodiment of the subject matter described herein;

FIG. 7 is a graph of experimentation results showing gamma radiationspectra for a standard count;

FIG. 8 is a graph of experimentation results showing gamma radiationspectra for a 4-inch operating mode;

FIG. 9 is a graph showing calibration curves for density measurements;

FIG. 10 is a graph of density measurements for various gamma-ray energybands as a glass thickness was varied;

FIG. 11 is a graph of the calibration curves for granite and limestonemixes as determined from experimentation;

FIG. 12 is a flow chart of an exemplary process for density measurementsin a backscatter mode using the gauge shown in FIG. 6 according to anembodiment of the subject matter described herein; and

FIG. 13 is a flow chart of an exemplary process for density measurementsin a transmission mode using the gauge shown in FIG. 3 according to anembodiment of the subject matter described herein.

DETAILED DESCRIPTION

The subject matter described herein includes methods, systems, andcomputer program products for measuring the density of a material and/orvarious other material properties. In one embodiment, the methods,systems, and computer program products described herein may determinethe radiation propagation properties of a material under test formeasuring the density of the material. According to one aspect, anuclear density gauge may include a radiation source for positioning inan interior of a sample material, such as soil. The radiation source mayemit radiation from the interior of the sample material for detection bya radiation detector. The radiation detector may produce a signalrepresenting an energy level of detected radiation. The nuclear densitygauge may also include a material property calculation functionconfigured to calculate a value associated with the density of thesample material based upon the signals produced by the radiationdetector.

In another embodiment, the methods, systems, and computer programproducts described herein may determine the radiation propagation andmoisture properties of a material under test for measuring the densityof the material. The material may be a construction related materialsuch as soil or asphalt or concrete. In one aspect, a material propertygauge may include a radiation source positioned for emitting radiationinto a material under test. A radiation detector may detect radiationfrom the material and produce a signal representing the detectedradiation. A moisture property detector may determine a moistureproperty of the material and produce a signal representing the moistureproperty. The material property gauge may include a material propertycalculation function configured to calculate a property value associatedwith the material based upon the signals produced by the radiationdetector and the moisture property detector.

Initially, it is noted that there are two dominant interactingmechanisms with matter for gamma radiation with energies less than 1mega electronvolt (MeV). For gamma ray energies less than 0.1 MeV, thedominant interaction is photoelectric absorption (PE) wherein the entiregamma radiation energy is provided for ejecting an electron from theatomic orbit. For common elements found in construction materials, thedominant interaction for gamma radiation energies greater than 0.2 MeV,is Compton scattering (CS), the scattering of photons by electrons inthe atoms

To explain the photon interaction types, consider a nuclear densitygauge that includes a gamma radiation source for producing a parallelbeam of photons with discrete energies having a uniform distribution.The beam of photons are directed through a sample material. If thephoton interaction mechanism is essentially photoelectric absorption(depending on the reaction cross-section or probability, which isspecific to the sample material), some of the photons are lost from theradiation beam due to absorption. Because of the absorption, the photonenergy spectrum will vary from location to location in the samplematerial. Since cross sections are higher for low energy photons (i.e.,energy less than 0.1 MeV), the low energy part of the spectrum shows adecreasing response or dip. The spectrum dip increases as the effectiveatomic number of the material increases. If the photon interaction isessentially Compton scattering, the photon energy spectrum will varyfrom location to location in the material with a variation of counts orflux in the high energy portion of the spectrum. Counts decrease as theelectron density increases and vice versa and are mostly independent ofthe elemental composition of the sample material.

Since there is a unique relationship between electron density andmaterial density for most materials, the gamma radiation flux may beused for measuring material density. Gamma radiation flux decreases inan exponential manner with the increase in material density. Nucleardensity gauges according to the subject matter described herein areoperable to expose sample material to gamma radiation, determine photoncounts of radiation emitted from the sample material and within apredetermined energy level, and determine density of the sample materialbased upon the photon counts with the predetermined energy level. Inpractice, both photoelectric absorption and Compton scattering existwith some probability in the entire energy range. Therefore, the energyspectral features (i.e., features in the low energy and high energyportions) can be used to accurately measure the material density.

For a material with an effective atomic number Z and atomic mass A, theelectron density ρ_(e) is provided by the following equation (wherein ρrepresents the mass density, and N_(A) represents Avagadro's number):ρ_(e)=ρ(Z/A)N _(A)In general, (Z/A) for a majority of the elements in construction or roadpaving materials is 0.5. One notable exception is H, where Z/A isabout 1. When Z/A is assumed to be 0.5, the density can be determinedbased upon Compton scattering.

Soils used for construction and asphalt typically have distinctlydifferent elemental composition. For gamma radiation-based densitymeasurements, construction soil material and asphalt material may betreated as different classes of materials because of their differentelemental composition. For soil, because of the wide variance in watercontent, separate measurement of the water content may be used forimproving the accuracy of density measurements. In nuclear densitygauges, an electromagnetic-based system or a neutron-based system may beused for determining a moisture property of the sample material, such aswater content or other moisture content.

When using gamma radiation-based nuclear density gauges for densitymeasurements, the determination of material differences in samples canbe challenging. The material that effects gamma radiation propagation isthe elemental composition, or the amount of various chemical elementscomposing the sample material. The density precision demanded byindustry can be as high as 0.65%. Therefore, a minor deviation of Z/Afrom 0.5 may require correction to meet industry density precisionrequirements.

Table 1 below shows exemplary chemical elements in constructionmaterials and corresponding Z/A.

TABLE 1 Z/A for Exemplary Chemical Elements in Construction MaterialsElement Z A Z/A % Diff. H 1 1.00797 0.992   98.42 C 6 12.0115 0.500 −0.10 O 8 15.994 0.500    0.04 Na 11 22.98977 0.478  −4.31 Mg 12 24.3050.494  −1.25 Al 13 26.98154 0.482  −3.64 Si 14 28.086 0.498  −0.31 K 1939.098 0.486  −2.81 Ca 20 40.08 0.499  −0.20 Ti 22 47.9 0.459  −8.14 Mn25 54.938 0.455  −8.99 Fe 26 55.847 0.466  −6.89

Table 2 below shows exemplary chemical elements for three limestonemixes and three granite mixes used for hot mixed asphalt in the roadconstruction industry. It is notable that most of the limestoneaggregates have similar Z/A values, and most of the granite typeaggregates have a similar Z/A value. The differences of both values to0.5 are significant enough to meet industry demand.

TABLE 2 Z and Z/A for Limestone and Granite Aggregate Mixes % Weight %Weight Limestone Granite 1- 2- 3- 9- 10- 11- SiO₂ 0.254 0.333 0.3240.667 0.595 0.663 Al₂O₃ 0.0037 0.0054 0.0034 0.136 0.156 0.138 CaO 0.410.363 0.341 0.0369 0.0642 0.0393 Mg 0.0047 0.0053 0.032 0.0185 0.03730.0195 Na₂O 0.0006 0.0009 0.0007 0.0353 0.0403 0.0363 K₂O 0.0008 0.00130.0007 0.0291 0.01 0.0298 Fe₂O₃ 0.0023 0.0025 0.0027 0.0409 0.06580.0409 MnO 0 0 0 0.0008 0.001 0.001 TiO₂ 0.0007 0.0009 0.0006 0.00620.0086 0.0064 CO₂ 0.3232 0.2877 0.289 0.015 0.0022 0.017 P₂O₅ 0 0 0.00180.0016 0.0165 0.0021 Sum 1 1 0.9959 0.9873 0.9969 0.9933 Avg. Z 10.0110.004 9.935 10.314 10.413 10.343 Avg. Z/A 0.4996 0.4995 0.4995 0.49740.497 0.4973

In one embodiment of the subject matter described herein, a Cs-137 gammaradiation source is used in a nuclear density gauge for measuring thedensity of a sample material. However, other suitable gamma radiationsources with different primary energy levels may be employed, such as aCo-60, Ra-60, or any other suitable isotope gamma radiation source forexample. Gamma radiation interacting with a sample material may bemeasured by an energy-selective, gamma radiation detector, which may beoperable to detect gamma radiation in one or more predetermined energyspectrums. For example, an energy-selective scintillation detector maybe used, such as a sodium iodide (NaI) crystal mounted on aphotomultiplier tube (PMT) for detecting gamma radiation in apredetermined energy spectrum.

As stated above, a nuclear density gauge according to the subject matterdescribed herein may include a moisture property detector fordetermining a moisture property of a sample material, such as soil. Thepresence of a significant fraction of water or various other moisture insoil may require correction to manage an anomalous Z/A value ofhydrogen. The moisture content of the sample material may be measuredusing the moisture property gauge and used for correcting densitymeasurements obtained by a nuclear density gauge.

An exemplary moisture property detector is a neutron-based detector,which is sensitive to low energy neutrons from a material. An example isthe gas tube detector filled with a gas of He-3 and CO₂, known as anHe-3 tube. The low energy neutrons have interacted with the hydrogencontained in water in the material. The detector may count the number ofslow moving neutrons. The count of slow moving neutrons may correspondwith a moisture property of the material. Thus, a moisture property ofthe material may be determined based on, the neutron count.Neutron-based detectors are calibrated at a construction worksite,because the chemical composition of the soils containing hydrogen, andnot associated with water, may affect the measurement results.

Another exemplary moisture property detector is an electromagnetic-basedmoisture property detector. These detectors and their components includeresistance-measuring components, capacitance measuring components, timedomain reflectometry components, frequency domain components, antennas,resonators, impedance measuring devices, fringing field devices, andbroadband devices, such as monopoles for example. Exemplary techniquesfor use in determining moisture content include microwave absorptiontechniques, microwave phase shift techniques, capacitance techniques,volumetric/gravimetric water content techniques, reflection-basedtechniques, transmission-based techniques, impedance spectroscopytechniques, Gypsum block techniques, resistance techniques, frequencyand time domain techniques, and combinations thereof.

An electromagnetic device may measure the permittivity of a material anduse the dielectric constant and conductivity to estimate the density ofthe material. Electromagnetic techniques are sensitive to the chemicalcomposition of the material, because permittivity is a result ofmolecular bonding, soil chemistry, texture, temperature, water content,void ratio, shape, and history of the material. Fundamentally, theelectromagnetic fields respond to the “dipoles per unit volume” or thechemical composition per unit volume. Hence, within even a small area ofmeasurement, there may be significant changes in material propertiessuch as texture, water content, clay content, mineralogy, and gradation.As a result, the electromagnetic device may require frequentcalibration.

Nuclear techniques are also a function of chemical composition as aresult of photoelectric effects and extraneous hydrogen not associatedwith water. However, the errors associated with nuclear techniques arevery forgiving as compared to dielectric spectroscopy techniques. Forneutron water measurements, hydrogen bonding from other chemicalcompositions is also measured, such as soils that are heavy in mica,salt, iron oxide, etc.

Signals produced by a radiation detector and an electrical propertydetector may be used by a material property calculation function forcalculating a property value associated with a material. The signalproduced by the radiation detector may represent an energy level ofdetected radiation from the material. The signal produced by theelectrical property detector may represent a moisture property of thematerial. The calculated property value may be a density of thematerial. The calculation of the material property values by thematerial property calculation function may be implemented by a suitablyprogrammed processor or by any other functionally equivalent device,such as an application specific integrated circuit (ASIC) or a generalpurpose computer, having suitable hardware, software, and/or firmwarecomponents.

In one example, soil content and losses can be estimated by inspectingdielectric constant dispersion over a microwave bandwidth from DC to afew GHz. FIGS. 1A-1C are graphs illustrating examples of dielectricdispersion for a variety of soils. In particular, FIG. 1A shows acomparison of dielectric constants of clay material (cohesive soil) andnon-clay material (non-cohesive soil) over different frequencies. FIG.1B shows dielectric constant dispersion of several different types ofclays. FIG. 1C shows the dielectric dispersion of the conductivity anddielectric constant of cohesive soil.

Information regarding dielectric constant dispersion for known materialsmay be used in the subject matter described herein for selectingcalibration curves for radiation detectors and moisture propertydetectors. Further, the subject matter described herein may be acombination asphalt and soils gauge having operability to measureasphalt layers in a backscatter mode and soils in a transmission mode.Further, for example, a fringing field planar detector may be attachedto a bottom surface of the gauge for simultaneously measuringelectromagnetic density and nuclear density. In this mode, the nuclearcomponent can calibrate the electromagnetic detectors in the field forimproving the speed of access to a capacitance asphalt densityindicator.

The combination of a radiation source/detector and a moisture propertydetector according to the subject matter described herein may operate ina transmission mode and/or a backscatter mode. The moisture property maybe measured using a surface technique, direct transmission, downholetechnique, or a technique using a fringing field capacitors, time domainreflectometry (TDR), microwave reflection, microwave transmission, realand imaginary impedance measurements, phase shift, absorption, andspectroscopic analysis. The sensor may be physically integrated into thesurface instrument. Alternatively, the sensor may be a stand-alonemoisture sensor linked electronically to the surface gauge. An exampleof a stand-alone system is a moisture sensor integrated into a drillrod. In a use of this exemplary gauge, a drill rod and hammer may beused to punch a hole in the soil to make a pathway for insertion of thesource rod.

Nuclear density measurements may be used to obtain bulk density, whichmay be derived using the following equation (wherein p represents bulkdensity, M represents mass, V represents volume, M_(W) represents themass of water, and M_(S) represents the mass of soil):

ρ = M/V = (M_(W) + M_(S))/V = (M_(W)/M_(S) + M_(S)/M_(S))/V/M_(S)wherein dry density M_(S)N is provided by the following equation(wherein ρ_(d) represents the dry density):ρ_(d)=ρ/(1+w)

Alternatively, a measurement of the volumetric water content may bedetermined using the following equation (wherein θ represents thevolumetric water content, V_(w) represents the volume of water, andV_(t) represents the total volume):θ=V _(W) /V _(t)The volumetric water content may be converted to pounds per cubic foot(PCF), where it may be subtracted from the wet density moisturemeasurement provided by the following equation (wherein γ_(w) representsthe density of water in proper units):ρ_(d)=ρ−γ_(w)θ

Variables affecting the electrical response of soils include texture,structure, soluble salts, water content, temperature, density, andfrequency. The following equation provides a general relationship forvolumetric water content (wherein s represents permittivity,A=−5.3×10⁻², B=2.92×¹⁰⁻², C=−5.5×10⁻⁴, and D=4.3×10⁻⁶):θ=A+B∈+C∈ ² +D∈ ³In this equation, permittivity E is the real part and is a single valuemeasured over the frequency content of the time domain signal. A similarequation may be found using the fringing field capacitor at a singlefrequency or over an average of frequencies. For results, the moisturedetector may be calibrated to the soil type directly from the field.

FIG. 2 is a vertical cross-sectional view of a nuclear density gauge 200for measuring the density of material according to an embodiment of thesubject matter described herein. Gauge 200 may be operable to accuratelydetermine the density of a sample material, such as soil, asphalt,concrete, or any other suitable construction and/or paving material. Forexample, soil may be measured in a transmission mode, and asphalt may bemeasured in a backscatter mode. Referring to FIG. 2, gauge 200 mayinclude a primary gamma radiation source 202 and a gamma radiationdetector 204. Radiation source 202 may be any suitable radiation source,such as a 300 micro Curie Cs-137 gamma radiation source. Gamma radiationdetector 204 may be any suitable type of detector, such as a gamma-rayscintillation detector of the type having a sodium iodide (NaI) crystal206 mounted on a photomultiplier tube 208. A gamma radiation detector ofscintillation-type is an energy selective detector. Radiation detector204 may be located adjacent to a base plate 210. When gamma radiationstrikes NaI crystal 206, photons are released, varying in intensitycorresponding to the energy level of the gamma radiation.Photomultiplier tube 208 detects the photons and converts them toelectrical signals which, in turn, are communicated to an amplifier foramplifying the electrical signals. Further, the amplified signals may bedirected, via an electrical conductor, to a printed circuit board (PCB)211, where the signals may be processed.

PCB 211 may include suitable hardware (e.g., a multi-channel analyzer(MCA)), software, and/or firmware components for processing theamplified signals. PCB 211 may include an analog-to-digital converterfor transforming the amplified analog signals into digital signalsquantifying the energy level of the gamma radiation (photon) energy. Theoutput of the analog-to-digital converter is directed to an analyzerdevice operable to accumulate the number of gamma radiation (photon)counts of different energy levels into a plurality of channels, eachchannel corresponding to a portion of the energy level spectrum. Forpurposes of density calculation, only a predetermined portion of theoverall energy spectrum detected by the detectors is considered. Thus,only the accumulated counts from one or more of the channelscorresponding to this predetermined portion are considered for thedensity calculation. The channel output may be used for densitycalculations, as described in further detail herein.

Gauge 200 may be adapted to position radiation source 202 in an interiorof a sample material 212 to be tested. For example, radiation source 202may be contained within a distal end of a movable, cylindrical sourcerod 214, which is adapted to be moved in the vertical directionsindicated by arrows 216 and 218. Source rod 214 extends into a verticalcavity 220 in a gauge housing 222. Source rod 214 may be restricted tomovement in the vertical directions by guides 224, a support tower 226,and an index rod 228. Guides 224 may include bearings that areoperatively positioned to guide source rod 214 through cavity 220 ingauge housing 222. Source rod 214 may be vertically extended andretracted to a plurality of predetermined source rod positions so as tochange the spatial relationship between radiation source 202 anddetector 204. The plurality of predetermined source rod positions mayinclude a backscatter position and a plurality of transmissionpositions, wherein radiation source 202 is positioned below base plate210 of gauge housing 222.

Index rod 228 may be operatively positioned adjacent to source rod 214for extending and retracting source rod 214. Index rod 228 may include aplurality of notches 230. Each notch 230 corresponds to a predeterminedsource rod position. For example, one notch may correspond to a “safe”position wherein radiation source 202 is raised and shielded from thesample material. Gauge 200 is shown in the safe position in FIG. 2. Thesafe position may be used to determine the standard count in abackground measurement mode, as described herein. Another notch maycorrespond to the backscatter mode wherein radiation source 202 islocated adjacent to the surface of the sample material underlying gauge200. Index rod 228 may include a flat side where a resistive depth strip(not shown) may be affixed. Other exemplary depth indicators includeHall effect devices, laser position indicators, and mechanical positionindicators.

Source rod 214 may be affixed to a handle 232 for manual verticalmovement of source rod 214 by an operator. Index rod 228 extends into acavity 234 in handle 232. Handle 232 further includes an indexer 236operatively positioned for engaging notches 230 of index rod 228 inorder to temporarily affix source rod 214 in one of the predeterminedpositions. Indexer 236 is biased into engagement with notches 230. Inparticular, indexer 236 may be biased into engagement by a spring 238. Atrigger 240 allows the operator to move indexer 236 into and out ofengagement with notches 230.

Source rod 214 may be positioned in a safe position as shown in FIG. 2and secured for positioning source 202 within a safety shield 242. Whenin the safe position, safety shield 242 contains the gamma rays emittedby source 202 minimizes the operator's exposure to radiation. Safetyshield 242 may be made of tungsten, lead, or any other suitableradiation shielding material.

Gauge 200 may also include additional shielding for preventingundesirable emission of gamma radiation from gamma radiation source 202.A stationary shield 244, safety shield 242, and a sliding block shield246 may be included within gauge 200 and positioned for stopping emittedphotons from directly reaching detectors of gauge 200. Shields 242 and246 may be made of tungsten. Alternatively, shields 242, 244, and 246may be made of any other suitable shielding material. Safety shield 242may include a hole formed therein and through which rod 214 and source202 may pass. In the safe position, source 202 may be positioned in theinterior of safety shield 242 for preventing photons of source 202 fromreaching the detectors of gauge 200. Stationary shield 244 may bepositioned for preventing photons from reaching the detectors throughpathways through the interior of gauge 200.

Detector 204 may be energy-calibrated by use of another gamma radiationsource 248. Radiation source 248 may be positioned within an aluminumsupport 249 and positioned adjacent to base plate 210 and detector 204.In one example, radiation source 248 may be a 1 to 2 micro Curie Cs-137gamma radiation source. Radiation source 248 may be used to energycalibrate detector 204 for managing environmental effects, such astemperature. In one example, radiation source 248 may produce mainenergy peaks of about 33 and 662 kilo electronvolts (keV). The energypeaks produced by radiation source 248 may be used for calibratingdetector 204 for use as a multi-channel spectrum analyzer, as describedin further detail herein. In an alternative embodiment, a small leakhole may be provided in cylindrical shield 242 to allow the energy fromgamma radiation source 202 to radiate towards detector 204 forenergy-calibrating detector 204.

Further, gauge 200 may include a moisture property detector 250 operableto determine a moisture property of sample material 212. In particular,detector 250 may measure the permittivity of sample material 212 and usethe dielectric constant and conductivity to estimate the moistureproperty of sample material 212. The following exemplary moistureproperties, alone or combinations thereof, may be detected by a moistureproperty detector for use in determining the density of a samplematerial: permittivity, resistivity, dielectric constant, conductivity,permeability, dispersive properties, change in dielectric constant withfrequency, change in conductivity with frequency, the real part ofpermittivity (i.e., dielectric constant), the imaginary part ofpermittivity, and combinations thereof.

In this example, moisture property detector 250 may include a moisturesignal source 252, a moisture signal detector 254, and a PCB 256.Moisture property detector 250, as an electromagnetic detector, mayoperate in a far field radiation mode, a near field mode, a passivefringing mode, or by coupling the fields from source to receiver throughsample material 212. Signal source 252 may generate an electromagneticfield and be positioned near a surface of sample material 212 such thatthe electromagnetic field extends into sample material 212.Alternatively, signal source 252 and/or detector 254 may be positionedwithin an interior of sample material 212 via source rod 214. In oneembodiment, a combination source/detector device may be attached to asource rod for obtaining depth information. In another embodiment, acombination source/detector device may be external to the gauge anddetached.

Moisture signal detector 254 may detect at least a portion of theelectromagnetic field from sample material 212 that was produced bysource 252. A frequency and/or time domain technique may be used fordetermining a moisture property. The electromagnetic field may rangefrom direct current (DC) to microwave. Exemplary techniques for use indetermining a moisture property include using fringing field capacitorsto produce an electromagnetic field; time domain reflectometrytechniques; single-frequency moisture techniques; sweeping-frequencymoisture techniques; microwave absorption techniques; and microwavephase shift techniques. Further, suitable moisture signal detectorsinclude detectors operable to measure the real and imaginary parts of adielectric constant at a single frequency, multiple frequencies,continuous sweeps of frequencies, and/or chirps of frequency content. Inthe time domain, direct steps or pulses may be produced by a signalsource and detected by a detector for determining a moisture property.In one example, source rod 214 may be pulsed, the response received atdetector 254, and the phase velocity calculated from the time-distanceinformation. Further, a fast Fourier transform (FFT) technique may beapplied to the frequency and time domains for determining a moistureproperty. The conductivity and permittivity of sample material 212 maybe determined based on the detected electromagnetic field.

Gauge 200 may include a source window 258 and a receiver window 260associated with signal source 252 and detector 254, respectively. Sourcewindow 258 and receiver window 260 may extend through base plate 210such that electromagnetic fields may pass through base plate 210 andbetween signal source 252 and detector 254. Exemplary window materialsinclude aluminum oxide, sapphire, ceramics, plastics, and suitableinsulators.

PCB 256 may be in operable communication with signal source 252 anddetector 254. PCB 256 may include suitable hardware, software, and/orfirmware components for control of signal source 252 and detector 254.In particular, PCB 256 may control signal source 252 to generate anelectromagnetic field. For example, PCB 256 may supply power tocircuitry of signal source 252 for generating a predeterminedelectromagnetic field. Further, PCB 256 may be operable to receive asignal from detector 254 representing detected electromagnetic fieldsvia a coaxial cable 262. Based on the signal representation, PCB 256 maydetermine a moisture property of sample material 212. For example,measurement of the magnitude and phase of reflected signals may providean impedance that is a function of constitutive parameters permittivityand permeability of the material. An impedance bridge may be used forobtaining the complex impedance at lower frequencies. For higherfrequencies, reflectometers incorporating mixers or detectors (e.g.,magnitude and phase integrated circuits, manufactured by Analog Devices,Inc. of Norwood, Masschusetts) may be used. For time domainreflectometry (TDR), diode techniques and timing/recording circuitry maybe used to obtain voltage as a function of time.

Other exemplary techniques for determining a moisture measurementinclude measuring a DC resistivity, surface impedance methods,propagation techniques, wave tilt, self-impedance, probe impedance,mutual impedance, transient electromagnetic methods, laboratoryresistivity methods, capacitance methods, transmission line methods,waveguide methods, free space methods, and mm wave and microwave remotesensing.

Moisture measurement may rely on single variable or multi-variableequations. For example, water may be detected using one variable such asthe relative dielectric constant ∈_(r). Interfacial polarization is animportant property response for heterogeneous materials. Further, therelaxation frequency of some soils is on the order of 27 MHz. At lowerfrequencies, the measured dielectric constant has the effects of theMaxwell Wagner phenomenon leading to errors in the water measurementthat are also a function of temperature. Other exemplary variablesinclude conductivity, permittivity, and the disperson of the change inconductivity and the change in permittivity with frequency. Further, forexample, the relaxation frequency of some soils is on the order of 27MHz.

In one example, the capacitance of a fringing field detector is measuredusing a feedback loop in an oscillator circuit. The frequency isprovided by the following equation (wherein C_(eff) represents theeffective capacitance including the surrounding medium, parasitics inthe circuitry, and nominal capacitances in the tank circuit, and Lrepresents the inductance):2πF=1/(sqrt(LC _(eff)))The ratio between a reference frequency and the frequency with thefringing field capacitor switched may be calibrated against moisture.The sensitivity of the measurement at these frequencies due to saltconcentrations should be considered. The end result is that chemicalcomposition errors must be corrected, leading to many differentcalibration curves for the soil types. Further, discussion is providedin U.S. Pat. Nos. 4,924,173; and 5,260,666, each of which areincorporated herein by reference in their entireties.

Microwave-based moisture property detectors may be advantageous, forexample, because such detectors can perform density-independent moisturemeasurements. Such detectors may be advantageous over neutron-basedmoisture property detectors, because neutron-based detectors are densitydependent. Further, it is desirable to reduce the use of neutron sourcesbecause of NRC regulations and fees associated with neutron sources.

Density-independent moisture measurements may be made based on atwo-parameter measurement of attenuation (or magnitude) and phase shiftin a transmission- or reflection-type mode. Alternatively,density-independent moisture measurements may be made using microwavesat a single frequency.

A two-parameter method may be implemented by comparing the real andimaginary parts of the dielectric constant, as shown in the followingequation (wherein E represents the dielectric constant):∈=∈(ω)′−j∈(ω)″

A density independent calibration factor A(ψ) (wherein ψ is thewet-based volumetric water content) may be used for canceling densitycomponents. The principle of density-independent moisture measurementsis based on both the real and imaginary part of the dielectric constantbeing related to dry material and water constituents, which change as afunction of density. Density components may be empirically canceled bycombining ∈(ρ_(d), ψ)′ and j∈(ρ_(d), ψ)″ in the following equation:

${A(\psi)} = \frac{{ɛ\left( {\rho_{d},\psi} \right)}^{\prime} - 1}{{ɛ\left( {\rho_{d},\psi} \right)}^{''}}$The above equation assumes that ∈(ω)′ and ∈(ω)″ are linearly independentfunctions of ρ_(d) and ψ.

The loss tangent ∈′/∈″ may describe the material interaction andresponse. The behavior of the complex permittivity implies thatnormalizing both ∈(ω)′ and j∈(ω)″ with density may reduce densityeffects. Further, data pairs may be normalized with bulk density asfunctions of temperature and moisture content. The following equationprovides a measure of bulk density without prior knowledge of moisturecontent or temperature given that moisture density relationships areindependent (wherein a_(f) represents slope, k represents intercept,a_(f) is related to the frequency, and k related to the dry dielectric):∈″/ρ=a_(f)(∈′/ρ−k)Alternatively, the following equation provides a measure of bulkdensity:ρ=(a _(f)∈′−∈″)/ka _(f)

At high frequencies, water is the dominant factor associated with energyloss related to ∈″ in the material, and the energy storage is related to∈′. Thus, a density-independent function for water content is based onthe loss tangent ∈″/∈′. Therefore, again, by normalizing the losstangent by the density provided by the above equation results in thefollowing equation:ξ=∈″/(∈′(a _(f)∈′−∈″))Here, the constant ka_(f) is omitted, and the loss tangent has beennormalized, resulting in a moisture function with reduced densityeffects. Experimentally, for granular materials, it has been found that√ν is linear with moisture content. ka_(f) is a function of themeasurement frequency and remains constant for data pairs of ∈′ and ∈″when they have been normalized by density.

Based on experimental results, it can be shown that, as temperatureincreases, the bound water becomes easier to rotate and the dielectricconstant increases. Thus, for the water measurement, temperaturecorrection may be necessary.

Since ν is a function of moisture content with the density effectsremoved, and since it is experimentally found to be linearly related tomoisture, calibration as a function of moisture and temperature can beimplemented by fitting to the following linear equation:√ν=A*M+B(T)In this equation, the intercept B increases with temperature, but theslope A is constant. For granular materials, the following equation wasempirically derived (wherein temperature is measured in Celsius):B(T)=9.77×10⁻⁴ *T+0.206The moisture content may then be determined using the followingequation:% M=(√ν(a _(f)∈′,∈″)−B(T))/AIn one embodiment, samples of soil may be extracted from the field andfit to this equation as a function of moisture yielding the constants Aand B at a particular temperature. Generic curves may also be definedwhereby a field offset is performed in use. Therefore, any moistureproperty detector operable to measure the real and/or imaginary portionsof the dielectric constant of a material at a single frequency, multiplefrequencies, or continuous sweeps of frequencies, chirps of frequencycontent, on the surface or down-hole can be incorporated intoembodiments of the subject matter described herein.

Microwaves are more sensitive to free water than bound water but arealso a function of the constituents of the chemical makeup of the drymass and water mass mixture. However, a dry mass and water mass mixtureis less susceptible to ionic motion and DC conductivity when consideringthe following equation:∈=∈(ω)′−j∈(ω)″=∈(ω)′−j(∈(ω)_(d)″+σ_(d.c.)/ω∈₀The higher frequencies reduce the effects of DC conductivity and measuremore of the dielectric permittivity. However, soil specific calibrationsmay be necessary. The differences in the calibrations are much smallerthan their low frequency counterparts. Thus, if the material changesslightly without a gauge operator's knowledge, suitable results maystill be obtained. Therefore, the microwave electromagnetic techniqueshave soil specific calibrations or offsets that may be required whencomparing sandy foams to clay classes of soils.

Sliding block shield 246 is configured to be slidable within a chamber264 and associated with a spring 266, which is adapted for biasingshield 246 in a direction towards an interior of safety shield 242. Inthe safe position, at least a portion of shield 246 is positioned in theinterior of safety shield 242 for preventing photons emitted by source202 from passing through safety shield 242. On movement of rod 214 inthe direction indicated by arrow 218 towards the position fortransmission mode, block shield 246 is pushed by an end of rod 214 awayfrom the interior of safety shield 242 and against the biasing directionof spring 266. Shield 246 may include a beveled portion 268 adapted toengage an end of rod 214 for pushing shield 246 away from the interiorof safety shield 242 such that rod 214 and source 202 may move into theposition for the transmission mode. Movement of shield 246 away from theinterior of safety shield 242 compresses spring 266.

FIG. 3 is a vertical cross-sectional view of nuclear density gauge 200configured in a transmission mode for measuring the density of samplematerial 212 according to an embodiment of the subject matter describedherein. Referring to FIG. 3, in the transmission mode, radiation source202 may be positioned in an interior of sample material 212 for emittingradiation from the interior of sample material 212. In the transmissionmode, radiation source 202 may emit radiation through sample material212 for detection by radiation detector 204. Further, PCB 211 mayproduce a signal representing an energy level of the detected radiation.Moisture property detector 250 may determine a moisture property ofsample material 212 and produce a signal representing the moistureproperty. A PCB 269 may include a material property calculation function(MPC) 270 configured to calculate a property value associated withsample material 212 based upon the signals produced by radiationdetector 204 and moisture property detector 250.

MPC 270 may include suitable hardware, software, and/or firmwarecomponents for implementing density measurement and calibrationprocedures according to the subject matter described herein. MPC 270 mayinclude one or more processors and memory components. Exemplary MPCcomponents include one or more of pre-amplifiers, spectroscopic gradeGaussian amplifiers, peak detectors, and analog-to-digital converters(ADCs) for performing the processes described herein. Procedure status,feedback, and density measurement information may be presented to anoperator via one or more interfaces of gauge 200.

A nuclear density gauge may be calibrated for density and moisturemeasurements. In one embodiment, measurements of the dielectricconstants of different synthetic materials are fit to a calibrationcurve. The materials may be selected to represent materials found in theconstruction field. Solid metal blocks of known properties may be usedfor calibrating a nuclear density gauge. Exemplary metal blocks for usein calibration include a magnesium (Mg) block (MG), a Mg and aluminum(AD-laminated block (MA), an Al block (AL), and a Mg andpolyethylene-laminated block (MP). The MG, MA, and AL set may be usedfor density calibration. The MG and MP set may be used for moisturecalibration. It is noted that the gravimetric density of Mg is about 110pounds per cubic foot (PCF), Al is about 165 PCF, and MG and Al areabout 135 PCF.

For density measurements, when calibrating a nuclear density gauge forsoil measurements, typical soils are assumed to have a Z/A of 0.5. Toemulate Z/A=0.5, the gravimetric density values of the calibrationblocks ρ_(grav) may be normalized with respect to the Z/A value and usedwith gamma radiation counts to determine calibration coefficients. Acalibration model is provided by the following equation (wherein CR isthe count ratio for the test sample, ρ_(norm) is the normalized densityof the test sample, and A, B, and C are calibration coefficients):CR=A×e ^(−Bpnorm) −CSoil normalization constants are shown in Table 3 below.

TABLE 3 Soil Normalization Constants Block MG MA AL Normalization 0.9880.974 0.964 Constants

When calibrating a nuclear density gauge for asphalt measurements, thenormalized gravimetric density values are used. Asphalt normalizationconstants are shown in Table 4 below.

Asphalt Normalization Constants Block MG MA AL Normalization 0.988 0.9890.949 Constants

A direct gauge reading on a test material is relative to the Z/A valueused in the calibrations. For materials having significantly differentZ/A values, the gauge may be calibrated specifically for the material.

For moisture example of laboratory calibration, a soil sample may beremoved from a field site. The soil sample is dried in an oven accordingto ASTM standard 2216. Different amounts of water are added to the driedsoil, and the material is stored for a predetermined time period. Thesoils are then compressed into a coaxial cylinder. Next, a function ofthe water content and measurements of the permittivity are obtained as afunction of frequency over a broad band. The permittivity is recorded asa function of frequency and temperature. The coaxial cylinders are thenweighed and dried to obtain the actual water and density content. Forsingle frequency measurements, the permittivity may be normalized withdensity and corrected for temperature. The slope a_(f) may be foundusing the equations described above. Further, by using the equationsdescribed herein, the moisture equation may be derived and programmedinto the nuclear gauge for field use.

In field use, the calibration for specific materials is performed byfinding an offset to the gauge by comparing gauge readings to densityvalues as determined by a conventional method. For example, a sand conetechnique (ASTM standard D-1556) may be used for soils. In anotherexample, an operator may use the gauge to perform a measurement in thefield, and use an oven test according to the ASTM standard 2216 toevaporate the water and obtain the moisture content in volumetric orgravimetric units. The resulting value in this example may be used tooffset factory or laboratory calibration. In an example for asphalt, acoring and water displacement technique(ASTM standard D-2726) may beused.

In field calibrations, the nuclear density gauge may be positioned onthe soil. Typically, the soil is wet with different moisture contents.Measurements of the real part of the dielectric constant may be obtainedas a function of the water content. The response is fit to a linearequation, such as y=mx+b, wherein x is the response of the gauge. Thenuclear density gauge may be calibrated in steps similar to the stepsused for laboratory calibration, except for one or more of thefollowing, only the imaginary portion of the dielectric constant isused, only the capacitance of a detector is used, only the resistancemeasurement is used, only TDR is used, only frequency response is used,only the relative dielectric constant is used, and only dispersion datais used.

As stated above, the presence of a significant fraction of water orvarious other moisture in construction-type soil may require correctionto manage an anomalous Z/A value of hydrogen. The wet density of soil isprovided by the following equation (wherein WD represents the wetdensity of soil, GD represents the gauge density (mass per unit volume)from direct calibration, and M represents gauge moisture content (massof water per unit volume of moist soil)):WD=GD−( 1/20)MThese corrections to direct nuclear gauge readings improve the accuracyof the density estimate provided Compton scattering is the onlyinteraction mechanism for gamma radiation. Detected gamma radiation ofenergies greater than 0.15 MeV meets this requirement for typicalconstruction materials.

Gas ionization detectors, such as Geiger Mueller detectors, may be usedin nuclear density gauges for gamma radiation or photon counting. Suchdetectors have relatively higher detection efficiencies in the 0 to 0.2MeV range than in the 0.2 MeV or higher range but cannot accuratelydetect the color or energy of counted photons. The photon countsrecorded by such detectors also contain the attenuation effect of lowenergy gamma radiation from photoelectric absorption. The modeldescribed above for handling the Z/A effect may not be met. As a result,density accuracy may be compromised.

A scintillation detector is an energy-selective detector operable toselectively use gamma radiation energies above 0.15 MeV during gaugecalibration and measurement. The signal amplitude of a sodium iodidecrystal/PMT detector depends linearly on the detected photon energy. Ahistogram of the number of detected photons versus energy signalamplitude provides a gamma radiation spectrum. For a given photonenergy, the energy signal amplitude depends on the PMT signal gain andthe environmental temperature. Therefore, with no feedback control ofthe detector, the position of key features of the spectrum (i.e.,spectrum peaks) vary with time. When counts in a particular energywindow (or range) are required, spectrum stabilization techniques may beused to minimize the effects form short-term signal amplitudevariability, as described in further detail herein.

FIG. 4 is a flow chart illustrating an exemplary process by which gauge200 shown in FIGS. 2 and 3 may be initialized at the beginning of aworkday according to an embodiment of the subject matter describedherein. In this example, radiation detector 204 is calibrated for use asa multi-channel spectrum analyzer. Referring to FIG. 4, the processstarts at block 400. In block 402, a high voltage power supply that isconnected to radiation detector 204 is turned on. For example, gauge 200may include a battery 276 configured to supply power to radiationdetector 204. In block 404, a predetermined number of channels in theenergy spectrum of the radiation provided by radiation source 202 todetector 204 may be set. In this example, the number of channels in thespectrum is set to 512. In block 406, radiation source 202 is positionedfor emitting radiation. Radiation detector 204 may detect radiationemitted by radiation source 202. As stated above, radiation source 202may be Cs-137 gamma radiation source for producing energy peaks of about33 and 662 keV. The energy peaks produced by radiation source 202 may beused for calibrating detector 204. During calibration, source rod 214may be positioned in a safety mode such that radiation detector 204 isshielded from radiation source 202.

In block 408, an amplifier gain of radiation detector 204 is set to adefault value. Further, in block 410, a data collection time ofradiation detector 204 is set to a predetermined time period (e.g., 20seconds). In block 412, the process waits a predetermined time period(e.g., between about two and five minutes). After detector 204 haswarmed up, a radiation count is obtained from the underlying material.

Next, in blocks 414-420, an amplifier gain of radiation detector 204 maybe adjusted until a centroid channel is between 208 and 212. Theamplifier gain may be set such that the centroid of the 662 keV gammaradiation peak from Cs-137 is in the middle of the 208 to 222 channelwindow. As the gauge is used, depending on the environment, the centroidmay move in the acceptance window defined by channels 200 and 220. Prioruse for measurements, MPC 207 may verify that the centroid lies in thischannel window. If MPC 207 determines that the centroid lies outsidethis channel window, the centroid may be moved back to the mid area ofthe channel window defined by channels 208 and 212 in about 20 seconds,and a message may be displayed on a display screen 274 of gauge 200indicating the delay. In a typical use, the gain may need to be moved tocenter the peak approximately one or two times per day. During idletimes, MPC 207 may implement an active routine for changing gain.

In particular, in block 414, data is collected from radiation detector204. For example, radiation detector 204 may communicate acquired dataand communicate the data to MPC 270. MPC 270 may calculate the centroidchannel for the 662 keV energy peak (block 416). In block 418, it isdetermined whether the centroid channel is between 208 and 212. If it isdetermined that the centroid channel is not between 208 and 212, theamplifier gain is changed (block 420). Otherwise, if it is determinedthat the centroid channel is between 208 and 212 the process stops atblock 422. Now, radiation detector 204 is ready for measurements.

The centroid may move in the acceptance window during normal temperatureconditions in the field. Further, when the gauge is used on hot asphalt,the increase in temperature of the radiation detector can result in acentroid location being outside of the acceptance window. If thecentroid location is found to be outside of the acceptance window, thesystem gain may be adjusted to center the centroid at channel 210. Thesystem gain may be changed by adjusting either the gain of the shapingamplifier or the voltage supplied to the photomultiplier tube of theradiation detector.

A detected energy level is analyzed when the location of a predeterminedenergy level peak is within an acceptance window. For example, an energylevel peak of 662 keV must be within an acceptance window of betweenchannels 200 and 220 within a 512 channel spectrum. FIG. 5 is a flowchart of an exemplary process for determining detector counts within anenergy window defined by energy values Ei and Ef according to anembodiment of the subject matter described herein. The process of FIG. 5may be implemented after the gauge has been initialized, for example, bythe exemplary process of FIG. 4. Referring to FIG. 5, in block 500, adata collection time of radiation detector is set to a predeterminedtime period (e.g., 15 or 30 seconds). Next, in block 502, energy valuesEi and Ef are obtained. In block 504, data is collected from radiationdetector 204.

Next, in block 506, MPC 270 may calculate a centroid channel C2 for the662 keV energy peak. MPC 270 may determine whether the centroid channelC2 is between channels 200 and 220 (block 508). If the centroid channelC2 is not between channels 200 and 220, the process can adjust theamplifier gain of radiation detector 204 according to a process similarto that described with respect to blocks 414-420 of FIG. 4 (block 510).Otherwise, if the centroid channel C2 is not between channels 200 and220, the process proceeds to block 512.

In block 512, using a look-up table, a centroid channel C1 may be foundfor the energy level peak of 33 keV. Next, in block 514, MPC 270 maysolve for coefficients A0 and A1 for calibration equation E=A0+A1*C, afirst order energy calibration where C is the channel number. In block516, MPC 270 may solve for channel numbers Ci and Cf corresponding toenergy values Ei and Ef, respectively. MPC 270 may then find counts CWcorresponding to energy values Ei and Ef (block 518). CW is the totalcounts of channels C1 to Cf, where a count value is associated with eachchannel. Counts CW may be used for density calculation processes, asdescribed in detail herein. Since channel numbers are integer values,fractional channel numbers may be handled in a manner as ananalog-to-digital converter digitizes signals.

Typically, sample material contains natural radioactivity, such asnatural radio isotopes of K, U, and Th. When using a low activity gammaradiation source, the natural radioactivity manifests itself as noise.Since the signal-to-noise ratio is low and the magnitude of the noisevaries from material to material, a separate measurement of the noise(background) is required for maintaining the accuracy of themeasurement. Nuclear gauge 200 is shown in FIG. 2 configured in abackground measurement mode for managing noise. As stated above, in thisconfiguration, shields 242, 244, and 246 prevent gamma radiationproduced by radiation source 202 from reaching radiation detector 204.The gamma radiation reaching radiation detector 204 is produced bymaterial sample 212 (natural radioactivity or background) andstabilization source 248. Since the small stabilization source 248 ispositioned near radiation detector 204, the background spectrum can bemeasured with adequate accuracy. Background counts are not necessary for8 milli Curie Geiger-Mueller detector-based instruments, because thesignal-to-noise ratio is high.

Nuclear density gauge 200 is operable in a backscatter mode formeasuring asphalt layers. FIG. 6 is a vertical cross-sectional view ofnuclear density gauge 200 for measuring the density of asphalt layersaccording to an embodiment of the subject matter described herein. Inthe backscatter mode, source rod 214 is positioned such that radiationsource 202 is on a surface of an asphalt layer 600.

Components of a nuclear density gauge operable in a backscatter modewere used for demonstrating the functionality of its use as atransmission gauge. The gauge components were positioned on amagnesium/aluminum (Mg/Al) standard calibration block of size24″×17″×14″. The gauge components included a 300 micro Curie Cs-137gamma radiation source fixed on a source plate. The base of the gaugeincluded a gamma radiation detector having a NaI crystal mounted on aphotomultiplier tube. PC-based electronics were used for dataacquisition.

Further, a source plate was attached to a 0.25-inch thick 14″×14″aluminum mounting bracket having an open slot with screw hole positions.The aluminum plate was attached to the 17″×14″ side of each metalcalibration block. The source plate was also attached to the aluminumplate so that the source is 2″, 4″, 6″, 8″, 10″, and 12″ below the topsurface (a 24″×17″ surface) of the calibration block. Each radiationsource position is called an operating mode.

Standard metal calibration blocks made of Mg, Mg/Al, and Al were usedfor calibrating the gauge. A standard count was used to compensate forthe decrease of the gamma radiation count over time due to radioactivedecay and other variations. In this experiment, counts for the gaugeoperating in the backscatter mode and placed on the Mg block was used asthe standard count.

For gauge calibration, data was collected for each of the operatingmodes, wherein the radiation source is positioned at 2″, 4″, 6″, 8″,10″, and 12″ below the top surface of the calibration block. A fourminute count time was selected for the calibration of the six operatingmodes. The net counts in the energy range from 150 to 800 keV were used.Further, radiation spectra were taken on the Mg block, the Mg/Al block,and the Al block without the radiation source for obtaining gammaradiation background.

In backscatter mode experiments, the radiation source was positionedabout 2″ from the radiation detector and about 7″ from the radiationdetector. It is noted that, in actual use, the radiation source and theradiation detector are in a fixed position with respect to one another.For each operating mode position, the transmission mode was tested withthe radiation source near the detector in the Mg block, the Mg/Al block,and the Al block. For obtaining the standard count, the gauge wasconfigured in the backscatter mode with and without the gamma radiationsource being positioned on the Mg block. FIGS. 7 and 8 are graphs ofexperimentation results showing gamma radiation spectra for the standardcount and the 4-inch operating mode, respectively.

In one embodiment, the mathematical model used for calibrating a nucleardensity gauge is provided by the following equation (wherein, CRrepresents the count ratio, and A, B, and C represent calibrationconstants):CR=A*exp(−B*Density)−CCR is defined as the ratio of the net counts for a mode on a block ofdensity ρ to the net standard count. For example, for a 6″ transmissionmode on a Mg/Al block, the net count is the difference of the counts forthe gauge with the gamma radiation source on the block and the gaugewithout the gamma radiation source on the block. The net standard countis the difference of the counts for the gauge in the backscatter mode onthe Mg/Al block with the gamma radiation source and without the gammaradiation source. Table 5 below shows the calibration constants for thesix operating modes.

TABLE 5 Calibration Constants A B C  2-inch 2.046 0.024546 −0.02849 4-inch 1.503 0.020331 0.00624  6-inch 1.533 0.022028 0.009511  8-inch1.799 0.026834 0.003163 10-inch 2.1219 0.032571 −1.29E−06 12-inch 1.48540.033686 0.000386Tables 6 and 7 below show the density precision obtained based on thecalibration data for 20-second and 1-minute counts, respectively.

TABLE 6 Density Precision for a 20-Second Count Mg Mg/Al Al Mode Density1-sigma Density 1-sigma Density 1-sigma  2-inch 109.4 0.21 133.5 0.36162.6 0.69  4-inch 109.4 0.23 133.5 0.34 162.6 0.58  6-inch 109.4 0.25133.5 0.38 162.6 0.68  8-inch 109.4 0.27 133.5 0.48 162.6 0.97 10-inch109.4 0.33 133.5 0.66 162.6 1.67 12-inch 109.4 0.47 133.5 1.03 162.82.66

TABLE 7 Density Precision for a 1-Minute Count Mg Mg/Al Al Mode Density1-sigma Density 1-sigma Density 1-sigma  2-inch 109.4 0.13 133.5 0.21162.6 0.4  4-inch 109.4 0.13 133.5 0.19 162.6 0.31  6-inch 109.4 0.13133.5 0.2 162.6 0.36  8-inch 109.4 0.15 133.5 0.26 162.6 0.55 10-inch109.4 0.18 133.5 0.38 162.6 0.96 12-inch 109.4 0.28 133.5 0.63 162.71.66

FIG. 9 illustrates a graph showing calibration curves for densitymeasurements.

In one example of gauge 200 being used in the transmission mode, countsin the energy interval from 150 to 800 keV for all spectra are used fordensity calculation. In this example, count are normalized per 1-minute.For a 4-inch operating mode on an Mg block, the net count for Mg is341084. The net standard count is 2181382. Further, solving from theequation CR=A*Exp(−B*Density)−C, density is provided by the followingequation:Density=(−1/B)*Ln((Cr+C)/A)The calibration constants A, B, and C for the 4-inch mode may be usedfrom Table 5 above, which may be stored in a memory associated with MPG270. CR is provided by net count/standard count, which is 341084/2181482in this example. By using the above equation, MPC 270 may determine thatthe density is 109.4 PCF.

A nuclear density gauge according to the subject matter described hereinmay operate in a backscatter mode for quality control and qualityassurance testing of asphalt pavements. Since asphalt pavements aretypically built with multiple layers including different mixes andthicknesses, an accurate estimate of the density requires considerationof the chemical composition, surface roughness, and the thickness of thetest layer.

Thickness of the top layer of an asphalt pavement may be specific forthe road construction project. For thin asphalt layers, a densityreading of the top layer may depend on the material type and density ofother asphalt layers below the top layer. The gauge reading may becorrected if the bottom layer density is known accurately by usingfeatures observed for layer-on-layer measurements. This correctionmethod is referred to a nomograph method and described in the TroxlerElectronic Laboratories, Inc. manual for the Model 3440 surface moisturedensity gauge, produced by Troxler Electronic Laboratories, Inc., ofResearch Triangle Park, N.C., the content of which is incorporatedherein by reference in its entirety. The Troxler ElectronicLaboratories, Inc. Model 4640 density gauge is another exemplary gaugefor thin-layer measurements, which uses two detector systems and thefeatures observed for layer-on-layer measurements.

When a photon is Compton scattered by an electron, the energy of thephoton depends upon the scattering angle. When a gamma radiation sourceand detector are placed on a planar semi-infinite medium, the singlescattered photons for a given thickness have predetermined energies.Such energy windows may be determined experimentally using measurementsof known thickness layers of materials, such as layers of glass on anMg/Al calibration block. The following energy bands may be used tomeasure layers with thicknesses between 0.75″ and 2.5″:

240 to 400 keV: 0.75″ to 1.25″

220 to 400 keV: 1.25″ to 1.75″

200 to 400 keV: 1.75″ to 2.0″

180 to 400 keV: 2.0″ to 2.5″

A dual layer structure made with dissimilar materials was formed in thelaboratory by placing glass slabs on an Mg/Al standard size block. Next,gamma-ray spectra were acquired by placing a nuclear density gauge onglass. FIG. 10 illustrates a graph of density measurements for variousgamma-ray energy bands as the glass thickness varied. The upper energyof all bands was 400 keV. By using the energy band 80 to 400 keV, thegauge measured a depth of about 3 inches. By using another energy bandfrom about 240 to 400 keV, the gauge measured a depth of about 1 inch.

When reading the density of thick layers, the window counts for densitydetermination contain gamma radiation of low energies. Such gammaradiation is also absorbed by the photoelectric process to thereby causean error in density. The two major classes of aggregate types, graniteand limestone, have two different normalization constants for gammaradiation in the Compton scattering region and varying degrees ofphotoelectric absorption. As a result, the granite and limestoneaggregate types have distinct calibration curves. FIG. 11 illustrates agraph of the calibration curves for granite and limestone mixes asdetermined from experimentation. A prior identification of aggregatetype can improve the estimation of the density.

MPC 270 may use the gamma radiation spectrum for identifying aggregatetypes. The photoelectric absorption process results in reducedlow-energy gamma radiation flux for materials with high atomic numbersthan that for materials with low atomic numbers. The average atomicnumber of limestone mixes is higher than that for granite mixes.Therefore, low energy counts in the spectrum normalized to density canbe used for aggregate type identification. For example, CL can representthe counts in a low-energy window with low and high energy limits(El_(l) and El_(h)), and CH can represent the counts in a high-energywindow with low and high energy limits (EH_(l) and EH_(h)). The ratio ofRc=CL/CH may be used for aggregate identification. Based on experiments,it was found that Rc<R0 for limestone mixes and that Rc>R0 for granitemixes.

In the asphalt industry, the asphalt volume for density determinationmay be defined in various ways. The material volume of the asphalt maybe determined by excluding surface texture. Further, a waterdisplacement technique and its variations may be used for densitymeasurements. Using gamma radiation techniques for density measurementsdefines the asphalt volume including surface roughness. Therefore,direct gamma radiation density values are lower than that measured bywater displacement techniques. Further, the air void content ofasphaltic materials (V) has a strong correlation to the surfaceroughness. If the density difference between the water displacement andgamma radiation techniques is dρ, an empirical relationship between dρand V may be found using the following equations:dρ=B0_(g) +B1_(g) *V+B2_(g) *V ² for granite, anddρ=B0_(l) +B1_(l) *V+B2_(l) *V ² for limestone.

Asphalt density measurements may be determined using gauge 200configured in the backscatter mode shown in FIG. 6. FIG. 12 is a flowchart illustrating an exemplary process for density measurements in abackscatter mode using gauge 200 according to an embodiment of thesubject matter described herein. Referring to FIG. 12, in block 1200,gauge 200 is positioned on a top surface of asphalt layer 600 as shownin FIG. 6. Further, source rod 214 is positioned in the backscatter modesuch that radiation source 202 is positioned near the top surface ofasphalt layer 600. Further, in the backscatter mode, sliding blockshield 246 is moved in the backscatter mode such that radiation source202 can emit radiation towards and into asphalt layer 600. An operatormay interface with gauge 200 to initialize a density measurement processin a backscatter mode for implementation by MPC 270.

In block 1202, a data collection time of radiation detector is set to apredetermined time period (e.g., between 15 and 30 seconds). Next, inblock 1204, energy values Ei and Ef for the energy window are obtained.The detector counts may be communicated to MPC 270 for use indetermining density of asphalt layer 600 in a backscatter mode.

In block 1206, steps similar to the steps described with respect toblock 504-518 may be implemented for determining low window counts CLand high window counts CH. As stated above, CL can represent the countsin a low-energy window with low and high energy limits (EL_(l) andEL_(f)), and CH can represent the counts in a high-energy window withlow and high energy limits (EH_(l) and EH_(h)).

In block 1208, MPC 270 may determine Rc ratio and count ratio CR. Theratio of Rc=CL/OH may be used for aggregate identification. The ratioCR=CH/Standard Count may be used for density determination.

In block 1210, MPC 270 may determine whether Rc is less than R0. Asstated above, Rc<R0 for limestone mixes, and that Rc>R0 for granitemixes. If it is determined that Rc is less than R0, a limestonecalibration curve is selected (block 1212). Otherwise, if it isdetermined that Rc is not less than R0, a granite calibration curve isselected (block 1214).

In block 1216, MPC 270 may determine raw density ρ using the limestonecalibration curve. Further, in block 1218, MPC 270 may determine voidcontent V. MPC 270 may also select a limestone calibration curve forsurface roughness (block 1220). In block 1222, MPC 270 may calculate adensity correction dρ. In one example, dρ may be determined by using oneof the above equations showing the empirical relationship between dρ andV. In block 1224, MPC 270 may determine the density of asphalt layer 600by adding raw density ρ and density correction dρ.

In block 1226, MPC 270 may determine raw density ρ using the granitecalibration curve. Further, in block 1228, MPC 270 may determine voidcontent V. MPC 270 may also select a granite calibration curve forsurface roughness (block 1230). In block 1232, MPC 270 may calculate adensity correction dρ. In block 1224, MPC 270 may determine the densityof asphalt layer 600 by adding raw density ρ and density correction dρ.

Soil density measurements may be determined in a similar manner to theasphalt density measurements. Some soils may have minerals having highatomic number elements, such as K and Fe. According to one embodiment,an energy-selective detector may be used for identifying soil type basedon features in the low-energy part of the spectrum. By using apredetermined calibration for the identified soil type, density errorsmay be reduced or avoided. Further, a correction to the gammaradiation-based density measurement may be made based on a determinedmoisture density. Soil density measurements may be determined usinggauge 200 configured in the transmission mode shown in FIG. 3.

FIG. 13 is a flow chart illustrating an exemplary process for densitymeasurements in a transmission mode using gauge 200 shown in FIG. 3according to an embodiment of the subject matter described herein.Referring to FIG. 13, in block 1300, gauge 200 is positioned as shown inFIG. 3 on a top surface of sample material 212, which is soil in thisexample. Further, source rod 214 is positioned in a transmission modesuch that radiation source 202 is positioned in the interior of soil 212within a vertical access hole 278 formed in soil 212. In thetransmission mode, gamma radiation emitted by radiation source 202 candirectly transverse through soil 212 to radiation detector 204. Anoperator may interface with gauge 200 to initialize a densitymeasurement process in a transmission mode for implementation by MPC270.

In block 1302, a data collection time of radiation detector is set to apredetermined time period (e.g., between 15 and 30 seconds). Next, inblock 1304, energy values E_(l) and E_(h) for the energy window areobtained. The detector counts may be communicated to MPC 270 for use indetermining density of soil 212 in a transmission mode.

In block 1306, steps similar to the steps described with respect toblock 504-518 may be implemented for determining low window counts CLand high window counts CH. As stated above, CL can represent the countsin a low-energy window with low and high energy limits (EL_(l) andEL_(f)), and CH can represent the counts in a high-energy window withlow and high energy limits (EH_(l) and EH_(h)).

In block 1308, MPC 270 may determine Rc ratio and count ratio CR. Theratio of Rc=CL/CH may be used for aggregate identification. The ratio ofCR═CH/Standard Count may be used for density determinations.

In block 1310, MPC 270 may identify a soil type of soil 212 based on thevalue of Rc.

Based on the identified soil type, a raw density ρ of soil 212 may bedetermined using a calibration curve corresponding to the identifiedsoil type (block 1312). MPC 270 may be operable to determine the rawdensity ρ using the calibration curve. The calibration curves forvarious soil types may be generated based on calibration blockcalibrations. As stated above, exemplary calibration blocks include Mg,Mg/Al, and Al.

Next, in block 1314, a moisture content M of soil 212 may be determinedusing moisture property detector 250. Moisture content may be determinedusing a neutron-based technique or an electromagnetic-based technique.

In block 1316, MPC 270 may determine density correction dρ. Densitycorrection dρ may equal the moisture content M/20. In block 1318, MPC270 may determine the density of soil 212 by subtracting densitycorrection dρ from the raw density ρ.

The calculated density value may be displayed to an operator via displayscreen 274. In one embodiment, the density calculation are carried outrepeatedly at frequency intervals as measurements are made, such asevery one to two seconds. Instead of waiting until the end of a 2 to 4minute count to display the density value, this approach makes itpossible to provide to the operator an almost real-time display of thecalculated density value while the count is still proceeding. Thedensity values may be displayed to the operator graphically as afunction of time. As the density value settles to a steady state, theoperator may decide to accept the calculated density value as beingsufficiently accurate, and to discontinue the measurement procedure.

The radiation source/detector and moisture property detector componentsmay be positioned in any suitable position in the interior or theexterior of a gauge. For example, a moisture signal source may bepositioned in an end of a source rod for generating an electromagneticfield from within an interior of a sample material. In this example, amoisture signal detector may be positioned within a gauge housing fordetecting the electromagnetic field transmitted through the samplematerial and generating a signal representing the detectedelectromagnetic field. Further, the generated electromagnetic field maybe an electromagnetic pulse or step. In another example, a moisturesignal source and detector may be attached to a drill rod operable topenetrate a sample material for positioning the moisture signal sourcein the interior of the sample material. In this example, the moisturesignal detector may generate a signal representative of detectedelectromagnetic fields, and communicate the signal via a wired orwireless communication connection to an MPC in a gauge housing.

A moisture property detector according to the subject matter describedherein may include one or more of several electromagnetic-basedcomponents. For example, the moisture property detector may include aduroid patch antenna configured to detect an electromagnetic fieldgenerated by an electromagnetic field source. The resonance frequency orinput impedance may be monitored as a function of a dielectric constant.

In another example, a moisture property detector may include acavity-backed dipole antenna. The antenna may include a dipole operableat predetermined frequency (e.g., 2.45 GHz). Further, the antenna mayinclude a metallic cavity filled with a dielectric material to decreasethe overall size of the component. The cavity may function as an energyfocus based upon the geometry of the cavity surface.

In another example, a moisture property detector may include a monopole.The monopole may detect broadband DC to microwave electromagneticfields. In use, the monopole may be driven by an oscillator. Theimpedance may be measured as a function of frequency and various soilparameters obtained. Alternatively, the impulse response can be obtainedand convolution and transform theory by be applied for obtaining soilproperties. Further, the monopole may be coated by an insulator toreduce the energy loss in the soil.

In yet another example, a moisture property detector may include asuitable fringing field, low-frequency device. The device may include asignal line, a ground, and one or more conductors.

It will be understood that various details of the subject matterdescribed herein may be changed without departing from the scope of thesubject matter described herein. Furthermore, the foregoing descriptionis for the purpose of illustration only, and not for the purpose oflimitation, as the subject matter described herein is defined by theclaims as set forth hereinafter.

1. A material property gauge for determining a property of a material,the material property gauge comprising: (a) a nuclear density gauge formeasuring the density of a material, the nuclear density gaugecomprising: a radiation source adapted to emit radiation into thematerial; a radiation detector operable to produce a signal representingthe detected radiation; and a first material property calculationfunction configured to calculate a value associated with the density ofthe material based upon the signal produced by the radiation detector;and (b) an electromagnetic moisture property gauge for determining amoisture property of the material, the electromagnetic moisture propertygauge comprising: an electromagnetic field generator configured togenerate an electromagnetic field including sweeping through one or morefrequencies and penetrating into a material, wherein the materialincludes at least one of a pavement material, aggregate base material,concrete, and a soil material; an electromagnetic sensor configured todetermine a frequency response of the material to the electromagneticfield across the one or more frequencies; and a second material propertycalculation function configured to correlate the frequency response to amoisture property of the material and to calculate a value representingthe moisture property; and (c) a third material property calculationfunction for determining a material property of the material based onthe value associated with the density of the material and the valuerepresenting the moisture property of the material.
 2. The materialproperty gauge of claim 1 wherein the electromagnetic sensor includesone of a monopole, resistance-measuring components,capacitance-measuring components, time domain reflectometry components,frequency domain components, antennas, resonators, impedance measuringdevices, fringing field devices, and broadband devices.
 3. The materialproperty gauge of claim 1 wherein the electromagnetic sensor includesdetectors operable to measure the real and imaginary parts of adielectric constant at one of a single frequency, multiple frequencies,continuous sweeps of frequencies, and/or chirps of frequency content.